Process for coating aqueous dispersion of epoxy resin and blocked polyisocyanate containing chemically incorporated anionic hydrophilic groups

ABSTRACT

The present invention is directed to an aqueously dispersed, heat-curable coating composition comprising 
     (a) a water-dispersible epoxy resin component containing external and/or chemically incorporated non-ionic emulsifiers and 
     (b) a water-dispersible, blocked polyisocyanate component containing chemically incorporated anionic hydrophilic groups in a weight ratio of component (a) to component (b) of about 1:10 to 10:1. 
     The present invention is also directed to a process for producing coated substrates from these compositions and to the coated substrates produced therefrom.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to aqueously dispersed, heat-curablecoating compositions containing a water-dispersible epoxy resincomponent and a water-dispersible blocked polyisocyanate component, to aprocess for producing coated substrates from these compositions and tothe coated substrates produced therefrom.

2. Description of the Prior Art

Epoxy resins which are rendered water-dispersible by external and/orchemically incorporated emulsifiers have recently increased inimportance due to the need to eliminate volatile organic solvents fromthese systems. Water-dispersible epoxy resin compositions containingnon-ionic and cationic emulsifiers are disclosed in U.S. Pat. Nos.4,048,179; 4,073,762; 4,304,700 and 4,315,044. The epoxy resincompositions also conventionally contain nitrogen-containing curingagents such as polyamines, polyamide resins or urea- ormelamine-aldehyde resins. Even though these curing agents provideadequate film properties, some of them are potentially physiologicallyharmful while others must be shielded from the atmosphere or it becomesdifficult to cure the compositions in which they are used into coatings.

It has surprisingly been discovered that the problems associated withthese known coating compositions may be overcome by usingwater-dispersible blocked polyisocyanates as curing or cross-linkingagents for epoxy resins made water-dispersible by external and/orchemically incorporated non-ionic emulsifiers. Water-dispersible blockedpolyisocyanates are known as disclosed by U.S. Pat. Nos. 4,098,933 and4,284,544; however, their primary utility has been as cross-linkers foraqueous polyurethane dispersions, polyester polyols or polyhydroxypolyacrylates.

It is an object of the present invention to provide aqueously dispersedcoating compositions based on epoxy resins and blocked polyisocyanateswhich are stable in storage, unaffected by the atmosphere and cure underthe influence of heat to form cross-linked coatings possessing improvedimpact resistance.

SUMMARY OF THE INVENTION

The present invention is directed to an aqueously dispersed,heat-curable coating composition comprising

(a) a water-dispersible epoxy resin component containing external and/orchemically incorporated non-ionic emulsifiers and

(b) a water-dispersible, blocked polyisocyanate component containingchemically incorporated anionic hydrophilic groups in a weight ratio ofcomponent (a) to component (b) of about 1:10 to 10:1.

The present invention is also directed to a process for producing coatedsubstrates from these compositions and to the coated substrates producedtherefrom.

DETAILED DESCRIPTION OF THE INVENTION

The water-dispersible epoxy resins used in the composition of thepresent invention have an average molecular weight of about 500 to20,000 and are prepared from a dihydric phenol and the diglycidyl etherof a dihydric phenol. In addition, an external emulsifier and/or achemically incorporated emulsifier based on a polyoxyalkylene glycol isused to provide hydrophilicity to the epoxy resin. Both the dihydricphenol and the diglycidyl ether of a dihydric phenol may also containother substituents such as alkyl, aryl, sulfido, sulfonyl, halo, etc.

Illustrative of suitable dihydric phenols are2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-bromo-4-hydroxyphenyl)propane,2,2-bis(3-chloro-4-hydroxyphenyl)-propane, bis(4-hydroxyphenyl)methane,bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfide, resorcinol,hydroquinone, and the like. The preferred dihydric phenols are2,2-bis(4-hydroxyphenyl)propane (bisphenol A) andbis(4-hydroxyphenyl)methane for reasons of cost and availability.

The diglycidyl ether derivatives are prepared by the reaction of adihydric phenol with a halogen-containing epoxide or dihalohydrin in thepresence of an alkaline medium. By varying the ratios of the dihydricphenol and epichlorohydrin reactants, different molecular weightproducts can be obtained as described in U.S. Pat. Nos. 2,582,985;2,615,007 and 2,633,458.

For purposes of the present invention, optionally at least a portion ofthe diglycidyl ether of dihydric phenol component can be replaced with adiglycidyl ether of a hydrogenated dihydric phenol derivative. Forexample, the said diglycidyl ether of dihydric phenol can have up toessentially 100 percent of its weight substituted by a diglycidylalicyclic ether such as 2,2-bis(4-hydroxycyclohexyl)propane orbis(4-hydroxycyclohexyl)methane.

In order to render the epoxy resins water-dispersible, either anon-ionic external emulsifier is added to the resin and/or a non-ionicemulsifier is chemically incorporated into the epoxy resin. Theemulsifiers contain repeating alkylene oxide units, preferably ethyleneoxide units, and have average molecular weights between about 400 and24,000.

Suitable external emulsifiers are disclosed in U.S. Pat. No. 4,073,762and include those of the alkylaryl type such as polyoxyethylene nonylphenyl ether or polyoxyethylene octyl phenyl ether; those of the alkylether type such as polyoxyethylene lauryl ether or polyoxyethylene oleylether; those of the alkyl ester type such as polyoxyethylene laurate,polyoxyethylene oleate or polyoxyethylene stearate; and those of thepolyoxyethylene benzylated phenyl ether type. In addition, reactionproducts of polyethylene glycols with aromatic diglycidyl compounds suchas those disclosed in U.S. Pat. No. 3,563,493 may also be used asexternal emulsifiers. The epoxy resin component may contain from about 1to 20%, preferably 2 to 15%, by weight of the external emulsifier, basedon the weight of the epoxy resin component.

Chemically incorporated non-ionic emulsifiers are based onpolyoxyalkylene glycols which are soluble or at least partially solublein water. Polyoxyalkylene glycols are prepared conveniently by thecondensation of an alkylene oxide with a suitable polyhydric alcohol.Illustrative of alkylene oxides are ethylene oxide and propylene oxideand mixtures thereof. Illustrative of polyhydric alcohols are aliphaticalcohols such as ethylene glycol, 1,3-propylene glycol, 1,2-propyleneglycol, 1,4-butylene glycol, 1,3-butylene glycol, 1,2-butylene glycol,1,5-pentanediol, 1,4-pentanediol, 1,3-pentanediol, 1,6-hexanediol,1,7-heptanediol, glycerol, 1,1,1-trimethylolpropane,1,1,1-trimethylolethane, hexane-1,2,6-triol, pentaerythritol, sorbitol,2,2-bis(4-hydroxycyclohexyl)propane, and the like.

Preferred polyoxyalkylene glycols are those prepared by the reaction ofethylene oxide and/or propylene oxide with a dihydric aliphatic alcohol,e.g., ethylene glycol. Illustrative of polyoxyalkylene glycols arecommercial Pluronic (BASF-Wyandotte) type products which are blockcopolymers of ethylene oxide and propylene oxide of about 5000-10,000molecular weight, containing from about 50 to about 90 weight percentethylene oxide and about 10 to 50 weight percent propylene oxide.

The polyoxyalkylene glycols may be chemically incorporated throughreaction of their hydroxyl groups with the epoxide rings of the epoxyresins as disclosed in U.S. Pat. No. 4,048,179. However, this method isnot preferred since it reduces the number of epoxide groups availablefor cross-linking with the water-dispersible blocked polyisocyanatecomponent of the present invention. Thus, it is preferred to convert thepolyoxyalkylene glycol into its diglycidyl ether prior to chemicallyincorporating it into the epoxy resin. These diglycidyl ethers may beconveniently prepared by reacting epichlorohydrin with a selectedpolyoxyalkylene glycol in a molar proportion which providessubstantially a diglycidyl ether reaction product. The epoxy resins maycontain from about 1 to 20%, preferably from about 2 to 15%, by weightof chemically incorporated polyoxyalkylene glycols or their diglycidylethers.

A preferred epoxy resin is the addition product of reactants comprising(i) about 50 to 90 parts by weight of the diglycidyl ether of a dihydricphenol, (ii) about 8 to 35 parts by weight of a dihydric phenol and(iii) about 2 to 15 parts by weight of the diglycidyl ether of apolyoxyalkylene glycol, wherein the average molecular weight of theepoxy resin is about 500 to 20,000.

The stable aqueous epoxy resin dispersions used according to the presentinvention may also contain a water-immiscible C₈ -C₂₀ aliphaticmonoepoxide reactive diluent component. The said monoepoxide componentcan contain alicyclic and aromatic structures, as well as halogen,sulfur, phosphorus, and other such heteroatoms.

Illustrative of monoepoxide reactive diluents are epoxidized unsaturatedhydrocarbons such as decene and cyclohexene; glycidyl ethers ofmonohydric alcohols such as 2-ethylhexanol, dodecanol and eicosanol;glycidyl esters of monocarboxylic acids such as hexanoic acid; acetalsof glycidaldehyde; and the like. The preferred reactive diluent is theglycidyl ether of monohydric C₈ -C₁₇ aliphatic alcohols.

The presence of a water-immiscible C₈ -C₂₀ aliphatic monoepoxidereactive diluent in an aqueous epoxy resin dispersion has a significantbeneficial effect on the properties of the aqueous dispersion. The saidwater-immiscible reactive diluent appears to function by coating theparticles of epoxy resin solids and thereby providing the aqueousdispersion with improved shear, freeze-thaw resistance, shelf viscositystability, and paint gloss.

Also, since the reactive diluent is epoxy functional, it may becomeincorporated into the film-forming substrate during the subsequent heatcuring of the aqueously dispersed composition after it has been blendedwith the blocked polyisocyanate and coated on a substrate. The totalquantity of reactive diluent contributes to the calculated proportion ofnon-volatiles in the dispersion composition.

The water-dispersible, blocked polyisocyanates used in conjunction withthe water-dispersible epoxy resins in the compositions of the presentinvention preferably contain an average of about 1-5, preferably about2-5, blocked isocyanate groups per molecule and may be prepared fromvirtually any organic polyisocyanate, preferably from polyisocyanatescontaining 2-4 isocyanate groups. Especially preferred arepolyisocyanates having aliphatically- and/or cycloaliphatically-boundisocyanate groups, although polyisocyanates having aromatically-boundisocyanate groups are not excluded and may be used.

The polyisocyanates used for preparing the water-dispersible blockedpolyisocyanates may be monomeric in nature or adducts prepared fromorganic diisocyanates and containing biuret, allophanate or urethanegroups or isocyanurate rings. Suitable polyisocyanates which may be usedas such or as intermediates for preparing polyisocyanate adducts includeethylene diisocyanate, 1,4-tetramethylene diisocyanate,1,6-hexamethylene diisocyanate, 2,4,4-trimethyl-1,6-hexamethylenediisocyanate, 1,12-dodecane diisocyanate, cyclobutane-1,3-diisocyanate,cyclohexane-1,3- and 1,4-diisocyanate and mixtures of these isomers,1-isocyanato-2-isocyanatomethyl cyclopentane,1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane, 2,4- and2,6-hexahydro tolylene diisocyanate and mixtures of these isomers, 2,4'-and/or 4,4'-dicyclohexyl methane diisocyanate, 1,3- and 1,4-phenylenediisocyanate, 2,4- and 2,6-tolylene diisocyanate and mixtures of theseisomers, diphenyl methane-2,4'- and/or 4,4'-diisocyanate,naphthalene-1,5-diisocyanate, triphenyl methane-4,4',4"-triisocyanate,polyphenyl polymethylene polyisocyanates of the type obtained bycondensing aniline with formaldehyde followed by phosgenation, andmixtures of the above-mentioned polyisocyanates.

Polyisocyanate adducts containing biuret groups may be prepared from thepreviously mentioned diisocyanates according to the processes disclosedin U.S. Pat. Nos. 3,124,605; 3,358,010; 3,644,490; 3,862,973; 3,903,126;3,903,127; 4,051,165; 4,147,714 or 4,220,749 by using coreactants suchas water, tertiary alcohols, primary and secondary monoamines, andprimary and/or secondary diamines. The preferred diisocyanate to be usedin these processes is 1,6-hexamethylene diisocyanate.

Polyisocyanate adducts containing allophanate groups may be prepared byreacting the previously mentioned diisocyanates according to theprocesses disclosed in U.S. Pat. Nos. 3,769,318 and 4,160,080, BritishPat. No. 994,890 and German Offenlegungsschrift No. 2,040,645.

Polyisocyanate adducts containing isocyanurate groups may be prepared bytrimerizing the previously mentioned diisocyanates in accordance withthe processes disclosed in U.S. Pat. Nos. 3,487,080; 3,919,218;4,040,992; 4,288,586; and 4,324,879; German Auslegeschrift No.1,150,080; German Offenlegungsschrift No. 2,325,826; and British Pat.No. 1,465,812. The preferred diisocyanates to be used are 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate mixtures of these isomers,1,6-hexamethylene diisocyanate isophorone diisocyanate and mixtures ofthe latter two diisocyanates.

Polyisocyanate adducts containing urea or preferably urethane groups andbased on the reaction product of the previously mentioned diisocyanatesand compounds containing 2 or more isocyanate-reactive hydrogens may beprepared according to the process disclosed in U.S. Pat. No. 3,183,112.In preparing polyisocyanate adducts the average isocyanate functionalityis determined from the functionality of the compounds containingisocyanate-reactive hydrogens. For example, when an excess of adiisocyanate is reacted with a diol, a polyisocyanate with afunctionality of 2 will be produced, while a triol co-reactant willresult in a polyisocyanate functionality of 3. By using mixtures ofcompounds containing isocyanate-reactive hydrogens, variousfunctionalities can be obtained. The preferred isocyanate-reactivehydrogens are provided by hydroxyl groups although amino groups are notexcluded. Suitable compounds containing isocyanate-reactive hydrogensare disclosed in U.S. Pat. No. 3,183,112, incorporated herein byreference, and include ethylene glycol, 1,2- and 1,3-propylene glycol,1,3- and 1,4-butane diol, 1,6-hexanediol, 2,2-dimethyl-1,3-propyleneglycol, glycerine, trimethylol propane, ethylene diamine, diethylenetriamine and triethylene tetraamine. 1,3- and 1,4-butane diol,trimethylolpropane and mixtures thereof are particularly preferred.Preferred diisocyanates are 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, mixtures of these isomers, 1,6-hexamethylene diisocyanateand isophorone diisocyanate.

The blocking agents which are suitable for preparing the blockedpolyisocyanates are compounds with preferably one isocyanate-reactivegroup which enter into an addition reaction with an isocyanate group attemperatures above about 50° C., preferably at temperatures in the rangeof from about 60° C. to 100° C., and wherein the resulting additionproduct, in admixture with the epoxy resins, reacts with hydroxyl groupsto form urethanes at temperatures in the range of about 100° C. to 250°C., the reaction being accompanied by liberation of the blocking agent.Suitable blocking agents of this type are, for example, secondary ortertiary alcohols, such as isopropanol or tert-butanol; C-H-acidcompounds, such as malonic acid dialkyl esters, acetyl acetone oracetoacetic acid alkyl esters; oximes, such as formaldoxime,acetaldoxime, methyl ethyl ketoxime, cyclohexanone oxime, acetophenoneoxime, benzophenone oxime or diethyl glyoxime; lactams, such asε-caprolactam, δ-valerolactam; phenols, such as phenol, cresol or nonylphenol; N-alkyl amides, such as N-methyl acetamide; imides, such asphthalimide; imidazole; or alkali metal bisulphites.

In order to make the polyisocyanates water-dispersible, it is necessaryto chemically incorporate a hydrophilic group, i.e., an anionic group ora potential anionic group, into the polyisocyanate component. Suitablehydrophilic components contain at least one isocyanate-reactive hydrogenand at least one hydrophilic group or potential hydrophilic group.Examples include aliphatic hydroxy carboxylic acids, aliphatic oraromatic aminocarboxylic acids with primary or secondary amino groups,aliphatic hydroxy sulfonic acids and aliphatic or aromatic aminosulfonicacids with primary or secondary amino groups. These acids preferablyhave molecular weights below about 300. It should be emphasized that theacid hydrogens are not considered to be isocyanate-reactive hydrogensdue to their sluggish reactivity with isocyanates.

The preferred anionic groups for use in the present invention arecarboxylate groups and these groups may be introduced by usinghydroxy-carboxylic acids of the general formula:

    (HO).sub.x Q(COOH).sub.y

wherein

Q represents a straight or branched, hydrocarbon radical containing 1 to12 carbon atoms, and

x and y represent values from 1 to 3. Examples of thesehydroxy-carboxylic acids include citric acid and tartaric acid.

The preferred acids are those of the above-mentioned formula wherein x=2and y=1. These dihydroxy alkanoic acids are described in U.S. Pat. No.3,412,054, herein incorporated by reference. The preferred group ofdihydroxy alkanoic acids are the α,α-dimethylol alkanoic acidsrepresented by the structural formula; ##STR1## wherein Q' is hydrogenor an alkyl group containing 1 to 8 carbon atoms. The most preferredcompound is α,α-dimethylol propionic acid, i.e., when Q' is methyl inthe above formula.

In order to prepare water-dispersible polyisocyanates containing about2-5 blocked isocyanates per molecule from a difunctional polyisocyanatestarting component such as a diisocyanate, it is necessary to usehydrophilic components containing at least two isocyanate-reactivehydrogens. For example, the use of a dihydroxy alkanoic acid to providehydrophilicity results in the linking of 2 diisocyanate molecules andmaintains the isocyanate functionality of the molecule at 2. To thecontrary, the reaction of a diisocyanate with a monohydroxy alkanoicacid produces a monoisocyanate. While a small portion of monoisocyanatesis acceptable since they may function as cross-linking agents asexplained in more detail below, in order to provide higher degrees ofcross-linking the isocyanate functionality should be maintained atbetween about 2 and 5.

The above-mentioned acid groups may be converted into hydrophilicanionic groups by treatment with a neutralizing agent such as an alkalimetal salt or a primary, secondary or preferably tertiary amine in anamount sufficient to render the blocked polyisocyanateswater-dispersible. Suitable alkali metal salts include sodium hydroxide,potassium hydroxide, sodium hydride, potassium hydride, sodiumcarbonate, potassium carbonate, sodium bicarbonate and potassiumbicarbonate. The use of alkali metal salts as neutralizing agents isless preferred than the use of volatile amines since they lead toreduced water resistance in the coatings produced from thewater-dispersible compositions of the present invention.

Examples of suitable amines are trimethyl amine, triethyl amine,triisopropyl amine, tributyl amine, N,N-dimethyl-cyclohexyl amine,N,N-dimethylstearyl amine, N,N-dimethylaniline, N-methylmorpholine,N-ethylmorpholine, N-methylpiperazine, N-methylpyrrolidine,N-methylpiperidine, N,N-dimethylethanol amine, N,N-diethyl-ethanolamine, triethanol amine, N-methyldiethanol amine, dimethylaminopropanol,2-methoxyethyldimethyl amine, N-hydroxyethylpiperazine,2-(2-dimethylaminoethoxy)-ethanol and 5-diethylamino-2-pentanone. Themost preferred tertiary amines are those which do not contain activehydrogen(s) as determined by the Zerewitinoff test since they arecapable of reacting with isocyanate groups during the curing of thecompositions of the present invention.

It is a preferred embodiment of the present invention that the tertiaryamines be volatile so that when the water-dispersible coatingcomposition of the subject application cures at elevated temperatures,the tertiary amine volatilizes, preferably at a lower temperature thanthe unblocking temperature of the blocked polyisocyanate, and is removedfrom the coated substrate. This frees the carboxylate or sulfonate groupto react with the epoxide ring of the epoxy resin component thusgenerating an additional hydroxyl group for subsequent cross-linkingwith an unblocked isocyanate group.

In order to prepare the water-dispersible blocked polyisocyanate thestarting polyisocyanate is preferably reacted at a temperature aboveabout 50° C., preferably at a temperature in the range of from about 60°to 100° C., with a quantity of blocking agent which corresponds to aratio of NCO-groups to NCO-reactive groups of from about 1:0.3 to1:0.95, preferably from about 1:0.50 to 1:0.85. However, it is alsopossible in principle to use an excess of blocking agent and to stop theblocking reaction at the required degree of blocking of from about 30 to95%, preferably from about 50 to 85%, of the isocyanate groupsoriginally present, by cooling to room temperature. In such a case, theexcess blocking agent is generally removed by distillation on completionof the reaction of the partially blocked isocyanate with the hydrophiliccomponents. The blocking reaction is usually carried out in the absenceof a solvent. It may be advisable to carry out the blocking reaction inthe presence of a catalyst, depending upon the type of blocking agentused. In cases where alcohols are used as blocking agent, it isadvisable to use a metal catalyst, for example dibutyl tin dilaurate. Incases where blocking agents containing activated methylene groups areused, it is advisable to use basic catalysts, such asdiazabicyclooctane, triethyl amine, alkali metal alcoholates or alkalimetal phenolates, such as sodium phenolate. The catalysts are used inquantities of from about 0.05 to 0.5%, by weight, based on the reactionmixture as a whole.

The free isocyanate groups still present on completion of the blockingreaction are reacted with the hydrophilic components in a secondreaction stage. The hydrophilic components are preferably used in such aquantity that there is at least one NCO--reactive group of thehydrophilic components for every isocyanate group still present.Reaction of the partially blocked polyisocyanate with the hydrophiliccomponents may be carried out in the presence or even in the absence ofsolvents. Suitable solvents include the water-miscible solvents normallyused in polyurethane chemistry such as esters, ketones, halogenatedhydrocarbons, alkanes and arenes. Low boiling solvents include thoseboiling at temperatures in the range of from about 40° to 90° C. such asacetone and methyl ethyl ketone. In addition, higher boiling solventssuch as N-methyl pyrrolidinone, dimethyl formamide, dimethyl sulfoxideand ethylene glycol mono(-methyl, -ethyl or -butyl) ether acetate may beutilized.

In one embodiment of the process according to the invention, forexample, solutions of the partially blocked polyisocyanate and thereaction components containing the hydrophilic group are combined atroom temperature or moderately elevated temperature (the hydrophiliccomponent may also be added as such to the solution of the partiallyblocked polyisocyanate), and kept at a moderately elevated temperature,for example, at a temperature in the range of from about 20° to 90° C.,until the addition reaction is over. On completion of the reaction, thedissolved end product may either be obtained as such by distilling offthe solvent (if a low boiling solvent is present) and any unreactedblocking agent still present, or, if there is no need to remove excessblocking agent, the end product may be converted into an aqueousdispersion by stirring the solution into water and subsequentlydistilling off the solvent (if a low boiling solvent is present). Whenhigher boiling solvents are used, they are maintained in the endproduct.

The acid groups may be converted into hydrophilic anionic groups bytreatment with the volatile amine either before, during or after thereaction of the hydrophilic component with the blocked polyisocyanate.However, it is preferred to neutralize the acid groups after thereaction of the blocked polyisocyanate with the hydrophilic componentand prior to dispersing it in water.

The aqueously dispersed coating compositions may be prepared from theepoxy resin and the blocked polyisocyanate by mixing each component,optionally in the presence of an organic solvent, with water followed bymixing the dispersions together or by mixing one component with waterfollowed by adding the remaining component. The coating compositionshould be formulated at about 5 to 80%, preferably 25 to 70% solids. Theweight ratio of the blocked polyisocyanate component to the epoxy resincomponent should be about 1:10 to 10:1, preferably about 1:2 to 2:1.

The aqueously dispersed coating composition of the present invention mayfurther contain additives conventionally employed in coating technology,such as organic pigments, inorganic pigments, surfactants, thickeners,and the like.

In normal usage after the aqueously dispersed composition is prepared itis coated on a substrate by brush, roller, spray, dip, doctor blade orsimilar application means.

The present invention provides aqueously dispersed compositions whichhave properties suitable for use as a one-package coating system inwhich the blocked polyisocyanate component is substantially unreactivewith the epoxy resin component at room temperature. In this type ofsystem, cross-linking is accomplished by baking an applied coating at anelevated temperature. For example, the present invention one-packagecoating system may be cured by heating a coated article at a temperaturebetween about 100°-250° C., preferably 100°-200° C., for a period oftime between about 5 minutes and two hours which is sufficient to effecta thermosetting cure.

The invention is further illustrated, but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLE 1

Into a one liter, three neck flask equipped with stirrer, thermometer,and condenser (fitted with drying tube filled with anhydrous calciumsulfate) was charged 360.0 g of the biuret of 1,6-hexamethylenediisocyanate (23.3% NCO content, Desmodur L-2291A, Mobay ChemicalCorporation). The material was stirred and to it was slowly added 130.6g methyl ethyl ketoxime such that the temperature was kept below 90° C.The mixture was stirred and the temperature was maintained at 90° C. forthree hours. Powdered α,α-dimethylol propionic acid (33.5 g) andN-methyl pyrrolidinone (184.9 g) were added and the mixture was stirredfor three hours while maintaining the temperature at 90° C. An infraredspectrum of the product showed that no residual isocyanate remained atthis time. The mixture was cooled to 68° C. and then 24.0 gtriethylamine and 50.0 g N-methylpyrrolidinone were added. Stirring wasmaintained for an additional half hour. The resulting product had aviscosity of 11,800 mPa.s at 23° C. and was readily dispersed in anequal portion of distilled water at room temperature.

EXAMPLE 2

Into a one liter, three neck flask equipped with stirrer, thermometer,and condenser (fitted with drying tube filled with anhydrous calciumsulfate) was charged 420.0 g of a 90% solution of the trimer of1,6-hexamethylene diisocyanate in ethylene glycol monoethyl etheracetate (20.0% NCO content and prepared in accordance with Example 1 ofU.S. Pat. No. 4,324,879). The material was stirred and to it was slowlyadded 130.6 g methyl ethyl ketoxime such that the temperature was keptbelow 90° C. The mixture was stirred and the temperature was maintainedat 90° C. for three hours. Powdered α,α-dimethylol propionic acid (33.5g) and N-methyl pyrrolidinone (150.6 g) were added and the mixture wasstirred for three hours while maintaining the temperature at 90° C. Aninfrared spectrum of the product showed that no residual isocyanateremained at this time. The mixture was cooled to 68° C. and then 24.0 gtriethylamine and 50.0 g N-methyl pyrrolidinone were added. Stirring wasmaintained for an additional half hour. The resulting product had aviscosity of 11,200 mPa.s at 23° C. and was readily dispersed in anequal portion of distilled water at room temperature.

EXAMPLE 3

A 55.6 g portion of CMD-35201 (non-ionic bisphenol A epoxy resindispersion, 60% solids in water, Celanese Plastics & SpecialtiesCompany) was further diluted with 10.0 g distilled water. Twenty dropsof a 5% aqueous solution of FC-430 (fluorocarbon surfactant, 3M Company)were stirred into the water dispersed epoxy. Drawdowns (10 mil wet filmthickness) were made on Bonderite® 100 steel panels* (6"×9", zincphosphate treatment). Most of the solvent was allowed to evaporate for30 minutes at ambient temperature and room humidity. The film was bakedfor 20 minutes at 140° C. A clear, defect-free film was obtained.Properties of the film are listed in the Table.

EXAMPLE 4

Equal portions of distilled water and the product described in Example 1were admixed. A 55.6 g portion of the epoxy resin dispersion used inExample 3 was mixed with 111.2 g of the water dispersed blockedpolyisocyanate. Twenty five drops of a 5% aqueous solution of FC-430(fluorocarbon surfactant, 3M Company) were stirred into the mixture.Drawdowns 10 mil wet film thickness) were made on Bonderite®100 steelpanels (6"×9" zinc phosphate treatment). Most of the solvent was allowedto evaporate for 30 minutes at ambient temperature and room humidity.The film was baked for 20 minutes at 140° C. A clear, defect-free filmwas obtained. Properties of the film are listed in the Table.

EXAMPLE 5

Equal portions of distilled water and the product described in Example 2were admixed. A 55.6 g portion of the epoxy resin dispersion used inExample 3 was mixed with 107.6 g of the water dispersed blockedpolyisocyanate. Twenty five drops of a 5% aqueous solution of FC-430(fluorocarbon surfactant, 3M Company) were stirred into the mixture.Drawdowns (10 mil wet film thickness) were made on Bonderite®100 steelpanels (6"×9", zinc phosphate treatment). Most of the solvent wasallowed to evaporate for 30 minutes at ambient temperature and roomhumidity. The film was baked for 20 minutes at 140° C. A clear,defect-free film was obtained. Properties of the film are listed in theTable.

EXAMPLE 6

A 55.6 g portion of the epoxy resin dispersion used in Example 3 wasfurther diluted with 10.0 g distilled water. Ethylenediamine (3.4 g) wasmixed into this solution. Twenty drops of a 5% aqueous solution ofFC-430 (fluorocarbon surfactant, 3M Company) were stirred into themixtue. Drawdowns (10 mil wet film thickness) were made on Bonderite®100steel panels (6"×9", zinc phosphate treatment). The solvent was allowedto evaporate for 30 minutes at ambient temperature and room humidity.The film was baked for 20 minutes at 140° C. A yellow, cloudy film withsevere bubbling was obtained. It was not further tested.

                                      TABLE                                       __________________________________________________________________________                                 Resistance to 1 hr spot                                        90°                                                                         Resistance to 100                                                                       Test (room temperature)                          Example #                                                                           Pencil Hardness                                                                       Bend Double MEK rubs                                                                         Water 50/50 water/isopropyl                      __________________________________________________________________________                                       alcohol                                    3     6B      no effect                                                                          completely removed                                                                      no effect                                                                           severe whitening                           4     4H--6H  whitens                                                                            no effect slight                                                                              slight whitening.sup.a                                                  whitening                                        5     4H--6H  whitens                                                                            no effect slight                                                                              very slight whitening.sup.a                                             whitening                                        __________________________________________________________________________     .sup.a Recovers within 24 hr. period                                     

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. An aqueously dispersed, heat-curable coatingcomposition comprising(a) a water-dispersible epoxy resin componentcontaining external and/or chemically incorporated non-ionic emulsifiersand (b) a water-dispersible, blocked polyisocyanate component containingchemically incorporated anionic hydrophilic groups in a weight ratio ofcomponent (a) to component (b) of about 1:10 to 10:1.
 2. The compositionof claim 1 wherein component (a) is the addition product of reactantscomprising a dihydric phenol and the diglycidyl ether of a dihydricphenol.
 3. The composition of claim 2 wherein at least a portion of saiddiglycidyl ether of a dihydric phenol is replaced with the diglycidylether of a hydrogenated dihydric phenol.
 4. The composition of claim 2wherein said diglycidyl ether of a dihydric phenol comprises thediglycidyl ether of bisphenol A.
 5. The composition of claim 2 whereinsaid dihydric phenol comprises bisphenol A.
 6. The composition of claim1 wherein said epoxy resin component contains a chemically incorporatednon-ionic emulsifier.
 7. The composition of claim 6 wherein saidchemically incorporated non-ionic emulsifier is the diglycidyl ether ofa polyoxyalkylene glycol based on ethylene oxide and optionallypropylene oxide.
 8. The composition of claim 1 wherein said epoxy resincomponent has an average molecular weight of about 500 to 20,000 and isthe addition product of reactants comprising (i) from about 50 to 90parts by weight of the diglycidyl ether of a dihydric phenol, (ii) fromabout 8 to 35 parts by weight of a dihydric phenol and (iii) from about2 to 15 parts by weight of the diglycidyl ether of a polyoxyalkyleneglycol based on ethylene oxide and optionally propylene oxide.
 9. Thecomposition of claim 1 or 8 wherein component (b) comprises the reactionproduct of(i) a polyisocyanate containing 2 to 4 isocyanate groups, (ii)a blocking agent for isocyanate groups in an amount sufficient to blockabout 30 to 95% of the isocyanate groups of component (i) and (iii) acompound containing at least one potential anionic group and at leastone isocyanate-reactive group in an amount sufficient to react with theremaining isocyanate groups of component (i), the potential anionicgroups being treated with a neutralizing agent in an amount sufficientto render component (b) water-dispersible.
 10. The composition of claim9 wherein component (i) is a biuret-containing polyisocyanate or apolyisocyanate containing isocyanurate rings.
 11. The composition ofclaim 10 wherein component (i) is prepared from 1,6-hexamethylenediisocyanate.
 12. The composition of claim 9 wherein said compound is adihydroxy alkanoic acid represented by the structural formula ##STR2##wherein Q' is hydrogen or an alkyl group containing 1 to 8 carbon atoms.13. The composition of claim 12 wherein Q' is methyl.
 14. Thecomposition of claim 9 wherein said anionic group is formed byneutralizing a sulfonic or carboxylic acid group with a tertiary amine.15. The composition of claim 14 wherein said tertiary amine is atrialkyl amine.
 16. The composition of claim 15 wherein said trialkylamine is triethylamine.
 17. The composition of claim 13 wherein saidalkanoic dihydroxy acid is neutralized with triethylamine.
 18. Thecomposition of claim 9 wherein said compound is a hydroxyl-groupcontaining sulfonic acid.
 19. The composition of claim 9 whereincomponent (ii) is methyl ethyl ketoxime.
 20. An aqueously dispersed,heat-curable coating composition comprising(a) a water-dispersible epoxyresin component having an average molecular weight of about 500 to20,000 and is the addition product of reactants comprising (i) fromabout 50 to 90 parts by weight of the diglycidyl ether of a dihydricphenol, (ii) from about 8 to 35 parts by weight of a dihydric phenol and(iii) from about 2 to 15 parts of a chemically incorporated emulsifier;and (b) a water-dispersible, blocked polyisocyanate component comprisingthe reaction product of (i) a polyisocyanate containing 2 to 4isocyanate groups, (ii) a blocking agent for isocyanate groups in anamount sufficient to block about 30 to 95% of the isocyanate groups ofcomponent (i) and (iii) a compound containing at least one potentialanionic groups and at least one isocyanate-reactive group in an amountsufficient to react with the remaining isocyanate groups of component(i), the potential ionic groups being treated with a neutralizing agentin an amount sufficient to render component (b) water-dispersible.
 21. Aprocess for forming a coating which comprises(a) coating a substratewith the coating composition of claim 1 or 20 and (b) heating saidcoated substrate to a temperature between about 100° C. and 250° C. fora time sufficient to cure said coating composition.
 22. The coatedsubstrate produced according to claim 21.